1. Field of the Invention
The present invention relates to an image display medium, and particularly relates to a reversible image display medium, in which image displaying and image erasing operations can be repeated.
2. Description of the Background Art
At present, image display is performed, e.g., in the following manners. A person uses a pencil, a pen, paints or the like, and manually writes or draws characters, pictures or the like on an image display medium such as paper sheet. Also, a computer, a word processor or the like is used to display text, graphics or the like on a display such as a CRT display, or output them on a medium such as a paper sheet via a printer for display.
A copying machine or the like may be used for producing duplication, on a medium of paper or the like, of the texts, pictures, graphics or the like, which are produced on the medium of paper or the like by a person or by a printer. A facsimile machine may be used for sending such contents (texts, pictures, graphics and others) prepared in the above manner for producing duplication on another medium of paper or the like.
The above image display, which is performed to display the texts, pictures or the like on the image display medium of paper or the like by a pencil, pen or the like, or by an image forming apparatus such as a printer, a copying machine or a facsimile machine operating in a electrophotographic method, an ink-jet method, a heat transfer method or the like, can achieve clear image display in a high resolution, and thus can achieve easy-on-the-eyes display.
However, it is impossible to repeat display and erasure of the images on the image display medium of paper or the like. In the case where the paper is used for writing characters or the like by a pencil, the characters can be erased by an eraser to a certain extent. However, it is difficult to erase completely the characters or the like written in an ordinary density, although it may be possible when written in a light density. The medium of paper or the like can not be reused except for the case of using the rear surface of the medium, which is not yet used for the image display.
Accordingly, the medium of paper or the like bearing images will be abandoned or burnt when it is not longer required. This results in consumption of a large mount of resources. The printer, copying machine or the like also consume consumable products or materials such as toner or ink. For obtaining the new display medium of paper or the like as well as toner, ink or the like, energies and resources are required for producing them. This is contrary to the current demand for reduction in environmental loads.
In contrast to the above, the image display by a display such as a CRT display can repeat the image display and the image erasure. However, the resolution, clarity and precision of images are restricted, as compared with the images displayed by the printer or the like on the paper medium or the like. Thus the image display by a display is improper especially when used for displaying the text documents mainly composed of letters because of low resolution. If it is used for displaying sentences which continue in less than the frame-size volume, it will do. However, if the sentences continue in twice or more times the frame-size volume, they may be difficult to read and to understand. Due to the relatively low resolution and the light emission from the display, operations for a long time are likely to be hard to eyes.
Electrophoretic display (EPD) and Twist ball-type display (TBD) have been proposed as an image display method allowing repetition of the image display and image erasure. Further displaying method was recently proposed, which is disclosed in “Japan Hardcopy '99, the book of the thesis, pp. 249-252”.
In the electrophoretic display method, two substrates including at least one transparent substrate are opposed together with a spacer therebetween to form a closed space therebetween, and the space is filled with a display liquid formed of a dispersion medium and electrophoretic particles, which are dispersed in the dispersion medium and are different in color from the medium. The image display is performed by an application of an electrostatic field and in a color of the particles or a color of the dispersion medium.
The display liquid is usually formed of isoparaffin-contained dispersion medium, particles of titanium dioxide or the like, dyes applying contrast in color to the particles, and an additive such as a surface active agent, or a charge applying agent.
In the electrophoretic display, the display is performed by utilizing contrast between particles of a high refractive index (e.g., titanium dioxide particles) and colored insulating liquid, and therefore the particles can not hide the colored liquid to a high extent, resulting in a low contrast.
Furthermore, there is a limitation on the kind of dye which is dissolved in a high concentration in a nonpolar solvent of high resistance which allows the electrophoresis of particles. A dye showing a white color is not found. Nor known is a black dye having a high extinction coefficient. Therefore the background portion becomes colored so that it is difficult to achieve a good contrast by a white background. When white particles for formation of images are placed into a colored liquid, the colored liquid may be moved between the substrate and the layer of white particles which are moved to the image observation side substrate, or the colored liquid may come into between the white particles, thereby lowering the contrast. The electrophoretic particles can scarcely uniformly adhere to the image observation side substrate, and thus the resolution is low.
Further, settling and condensation of particles are liable to occur due to a very large difference in specific gravity between the particles and the dispersion medium in the display liquid. This is liable to lower the display contrast. Further, it is difficult to display the images with high stability for a long time, and remaining of last images is liable to occur. Further, the degree of charging of the particles in the liquid significantly changes with time, which also impairs the stability of the image display.
In the twist ball-type display method, images can be displayed in specified colors using an image display medium containing numerous microcapsules enclosing not only an insulating liquid but also fine spheric particle so processed that a half of their surface and the other surface portion show colors or an optical density which differs from each other. Images are displayed in predetermined colors by rotating the fine spheric particles in the microcapsules due to an electric field strength or magnetic strength.
However, according to the twist ball-type display, images are displayed using fine spherical particles in the insulating liquid within the microcapsules. This makes it difficult to attain good contrast. Further, the resolution is low since spaces are formed between the microcapsules. In the manufacture of microcapsules, difficulty is entailed in reducing the size of microcapsules to increase the resolution.
The “Japan Hardcopy '99, the book of the thesis, pp. 249-252” discloses an image displaying method wherein a closed space is formed by placing two substrates as opposed to each other and as spaced from each other, i.e. the two substrates being a laminate of electrodes and a charge transporting layer, the space being used to enclose the electrically conductive toner and insulating particles which are different in color from the toner, an electrostatic field being applied to inject charges into the electrically conductive toner so that the toner is moved by a Coulomb force applied thereto to display images.
However, the foregoing image displaying method utilizing the charge injection phenomenon poses problems. When the electrically conductive toner carrying the injected charges is moved, insulating particles (e.g. white particles mixed together to form the color of background) interfere with the movement of the toner particles, making their movement so difficult that some of them may stop their movement. This results in failure to obtain satisfactory image density and good contrast and in reduction of image display rate. To overcome these problems, a high voltage drive is necessitated. The resolution is determined by the electrodes and is so limited. Furthermore, it is essential to use electrodes, charge-injection layer and electrically conductive toner, which results in limited manufacture.